/Engine/Source/Runtime/Core/Public/Misc/MapErrors.h
Volume LevelStreamingVolume is not in the persistent level - please delete: This warning is caused when there is a level streaming volume that does not exist in the persistent level. This can be problematic because the volume will not be considered when checking to see if a streaming level should be loaded or unloaded. You can fix this problem by deleting the level streaming volume and recreating it.
static [FLazyName](API\Runtime\Core\UObject\FLazyName) LevelStreamingVolume;